The DNA can have random mutations, but they are duplications of a gene, suppresions of a gene or little modifications of a gene. And even the mutations follow a pattern knowing the mecansim involved .
The apparition of an entire gene isnt random. This is fantasy but the inherancy of the X-gene is no just a biology matter is a relevant piece of canon about the origin of the mutants.
And that means that talking about mutant supremacy is not "inevitable evolution" is just eugenics.